Quote (Black XistenZ @ Mar 20 2022 08:42am)
Mariupol is located between the Sea of Azov and two Russian deployment areas on Crimea and in Donezk oblast. The city was always more difficult to hold than other large cities in Ukraine. Also, Mariupol is the only major city that the Russians were able to fully encircle, which they failed to do so far in Kyiv, Kharkiv, Sumy, Tchernihiv or Mykolaiv.
There might be some sort of domino effect though: once Mariupol falls, this frees up Russian forces in the South East which could help push back Ukraine's main regular military forces which are stationed on the battlefront of Donezk and Luhansk. Then, the joined up Russian forces from the south and the Donezk/Luhansk regions could apply pressure on Kharkiv from its south and south east, which in turn would reduce Ukrainian capabilities for divisionary counterattacks in the region, counterattacks which have up until now diverted Russian resources away from Kyiv's eastern flank. This, in turn, would make it more easy to close the encirclement of Kyiv.
Now, the Ukrainian high command is obviously aware of this threat; we can assume that they and their Western allies have been preparing for such a development.
ISW said they think the freeing up of Russian forces after the fall of Mariupol won't be significant in terms of tipping the scales of the war in the South. It makes sense somewhat, with Mariupol being a hostile city they'll need a lot of forces to hold it, plus the Russians have taken significant losses in the region so we don't know what proportion of forces could push up North. The urban fighting will cause even more losses over the coming weeks
Regarding your point about counterattacks on Kyiv's eastern flank, this is conducted by Ukrainian army and TDF in the Sumy region, towns like Romny that Russia don't even control
There isn't a way to sugarcoat that Southern Ukraine will most likely fall after Mariupol, the big question is whether the JFO can escape or at least prevent being encircled